Three decades later, Spirit Lake holds a new mystery: How did fish, now twice the length of those pre-eruption rainbows, reappear? Everyone has a theory. Smith, who runs Eco Park Resort at the edge of the volcanic monument, thinks the trout slid down from smaller, higher St. Helens Lake during a flood year. But that lake has only mackinaw—and the Spirit Lake fish are rainbows. Biologist Bob Lucas of the Washington Department of Fish and Wildlife believes someone illegally planted them. In the late 1990s, an anonymous call to his home seemed to confirm it: "I'm the one who stocked the fish." Preliminary genetic testing by Forest Service ecologist Charlie Crisafulli also suggests the trout did not descend from the pre-eruption population, but he's given up on figuring out their origin. "There are as many stories as there are fish tales," he says, "and all of them start, 'I know somebody who put those fish in there.' " To him the important question is not how they arrived but how they grew so big. On the 30th anniversary of the May 18 eruption, one of the only things certain about the trout in Spirit Lake is that they've given everyone—environmentalists, scientists, fishermen, congressmen, rangers, and business owners—something else to argue about.

Mark Smith grew up at the lake, where his family ran the lodge one down from the one owned by Harry Truman, the famously cantankerous 83-year-old who shared a name with a President and was among the eruption's 57 victims. As a boy, Smith fished there. Today he'd have to break the law to do so. He's not saying he does, but if ever I want to join him, he is, ahem, very familiar with where a poacher could sneak in. "We were lost!" he yells, practicing his alibi. "We just saw this and started fishing!" The 2,700-acre lake now sits at the center of a restricted research area taking up roughly a quarter of the 110,000-acre Mount St. Helens National Volcanic Monument, which Congress set aside in 1982 "to protect the geologic, ecologic, and cultural resources … in as natural a state as possible, allowing primarily natural geologic forces and ecological succession to continue unimpeded." Mostly closed to the public, this part of the blast zone has become one of our planet's grandest experiments.

The volcano came back to life from 2004 to 2008, shooting off plumes of steam and ash up to 30,000 feet into the sky, growing a new lava dome in its crater, and captivating sightseers and geologists. But many of the area's greatest insights have come in the field of ecology.

As a natural lab to study the rebirth of ecosystems, the blast zone has no equal. "It's the most thoroughly studied large-forest disturbance in the world," says Crisafulli, examined from nearly every angle, at nearly every scale, from molecules to ecosystems, bacteria to mammals, steaming geothermal vents to waterlogged meadows. Almost daily, callers inquire about the lessons of St. Helens. One woman is interested in salamanders, another in toads. Officials in Alaska and Chile want to know what to expect after eruptions of their own.